I'm pleased to say that with all those units that have been sold there have been no returns. We almost had one, here's the story.
0000 - 1FFF
Internal 5K RAM + CART RAM1/RAM2/RAM3
2000 - 3FFF
4000 - 5FFF
6000 - 7FFF
8000 - 9FFF
Video / IO space
A000 - BFFF
C000 - DFFF
E000 - FFFF
A cartridge can add ROM or RAM at blocks 1,2,3 and 5, and also 3K of RAM into block 0, as three 1K blocks at RAM1, RAM2 and RAM3 (hence the maximum = 4x8K + 3x1 = 35K). 8K was working, so was 11K, so block 0 and block 1 were OK, but as soon as RAM was added to block 2, it would fail. In order to narrow things down, I asked the user to try a few games. The VIC20 will automatically start a ROM with an appropriate signature when placed in block 5. Most cartridges use an 8K ROM in block 5, and many also have an additional 8K ROM in another block. There isn't a standard here, some use block 1 some block 3. I suggested testing the following games:
5 + 1
5 + 3
2 + 3
Avenger, Centipede and Defender all worked fine, so blocks 1, 3 and 5 were OK. Adventure Land required block 2, and that wasn't working. So again this was pointing to block 2. This exonerates the RAM chip, but points the finger at one of the following:
- Something in the cartridge responding incorrectly when block 2 was selected
- A bad contact on the edge connector carrying the block 2 signal
- Something inside the VIC20 causing an issue with block 2 access
The user had already cleaned the edge connectors, and they looked fine, which seems to rule out option 2, so the cartridge was return to TFW8B for testing. When received, it tested fine, so that rules out 1. The only candidate remaining was a fault in the VIC20. The user wasn't too far from me, so dropped his VIC20 off for further testing.
- Do you have a VIC20 that needs a repair?
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